The present invention relates to a circuit for driving inductive loads.
More specifically, the subject of the invention is a driving circuit comprising:
at least one electronic switch which is operatively connected substantially in series with an inductive load and, in an enabled condition, can connect the load to a direct-current voltage supply, the switch being intended to be driven by an on-off signal, PA1 at least one freewheeling circuit branch which comprises a diode and can recirculate the transient current which flows through the load each time the switch is disabled, and PA1 a filtering circuit connected between the cathode of the diode and the voltage supply and comprising a parallel capacitor connected to the cathode of the diode and a series inductor connected between the cathode of the diode and the voltage supply.
FIG. 1 of the appended drawings shows a diagram of a driving circuit of this type according to the prior art. In this drawing, an inductive load, such as the rotor winding of a direct-current electric motor with brushes, is indicated W. The driving circuit comprises an electronic switch Q, such as an n-channel MOSFET transistor, connected substantially in series with the load W, between the load and the earth GND. The terminal of the load W remote from the switch Q is connected to the positive terminal of a direct-current voltage supply such as a battery B, by means of a filtering circuit generally indicated FC. The driving circuit further comprises a freewheeling branch FB including a diode D having its anode connected to the terminal of the load W which is connected to the switch Q and its cathode connected to the other terminal of the load W.
The freewheeling circuit branch can limit the overvoltages which are created when the switch is opened and can restrict power losses as is required in the driving technique using a pulse-width modulated signal (PWM).
The filtering circuit serves to limit the amplitude of the variable components of the current in the supply line, as is required by the standards in force for equipment in motor vehicles in order not to transmit interference to other loads connected to the same supply line.
The most usual embodiment of the filtering circuit FC comprises a parallel capacitor C connected between the cathode of the diode D and the earth, a series inductor LF, and a capacitor CF substantially in parallel with the voltage supply B.
In the circuit according to the prior art shown in FIG. 1, the freewheeling circuit branch FB is connected in parallel with the load W. The parallel circuit comprising the load and the freewheeling circuit branch FB is connected substantially in series with the filtering circuit FC.
In operation, an on-off driving signal, for example, of the pulse-width modulated type (PWM), is applied to the control input (the gate) of the switch Q.
The topography of the circuit according to the prior art described above is such that, in operation, substantially the entire current which circulates operatively in the load W flows through the inductor LF of the filtering circuit FC.
This inductor has a resistive component and, in operation, it therefore dissipates a large amount of electrical power as heat. This leads to a reduction in efficiency in operation and to problems from the point of view of the physical location of the filter inductor LF which, in some applications, preferably has to be arranged remote from the rest of the circuit or has to have a suitable heat dissipator.